Structure-sensitive Reaction Research Articles

Influence of pretreatment on the catalytic performance of Ru/SBA-15 catalysts for glycerol hydrogenolysis

Ru/SBA-15 catalysts were prepared by impregnation and were pretreated under different conditions, and were characterized by N 2 adsorption-desorption, X-ray diffraction, CO chemisorption, transmission electron microscopy, and the reaction of glycerol hydrogenolysis. The catalyst calcined in air had larger Ru particles, while the direct reduction of the catalysts with H 2 without air pre-calcination made the Ru particles dispersed well. As the H 2 reduction temperature increased, the Ru dispersion decreased. The turnover frequency (TOF) of the catalysts increased as the Ru dispersion decreased, indicating that Ru/SBA-15 catalyzed glycerol hydrogenolysis is a structure sensitive reaction. Different treatment conditions gave Ru/SBA-15 catalysts with different Ru particle size, and they showed that glycerol hydrogenolysis over Ru/SBA-15 is a structure sensitive reaction.

Synthesis and Electrocatalytic Properties of H2SO4‐Induced (100) Pt Nanoparticles Prepared in Water‐in‐Oil Microemulsion

The increasing number of applications for shape-controlled metal nanoparticles (NPs) has led to the need for easy, cheap, and scalable methodologies. We report the synthesis of (100) preferentially oriented Pt NPs, with a particle size of 9 nm, by using a water-in-oil microemulsion method. The specific surface structure of the NPs is induced by the presence of H(2)SO(4) in the water phase of the microemulsion. Interestingly, the results reported herein show how increasing amounts of H(2)SO(4) lead to the formation of Pt NPs containing a larger amount of (100) sites on their surface. This preferential surface orientation is confirmed electrochemically by using the so-called hydrogen adsorption/desorption process. In addition, transmission electron microscopy measurements confirm the presence of cubic-like Pt NPs. Finally, the electrocatalytic properties of the Pt NPs are evaluated towards ammonia and CO electro-oxidations, which are (100) structure-sensitive reactions.

Density functional theory study on the adsorption and decomposition of H2O on clean and oxygen-modified Pd (1 0 0) surface

Based on density functional theory together with periodic slab models, the adsorption and the corresponding dehydrogenation reaction of H2O on clean and oxygen modified Pd (1 0 0) have been investigated. The preferential sites for H2O, OH, O, and H were obtained on the surfaces. According to the optimized structural and energetic properties, it was found that H2O prefers to adsorb on the top site with weak adsorption energy (physisorption), whereas O and H atoms are prone to adsorb on the hollow site and OH occupies the bridge site. In addition, this work identified the optimum configurations for the relevant co-adsorption groups. The results confirmed that co-adsorption tends to weaken the adsorbate-substrate interaction due to the existence of oxygen atom, and that the OH group, O and H atoms are less stable on oxygen-covered Pd (1 0 0) surface than on the clean surface. Finally, the transition states and related barrier energies were ascertained to analyze the dehydrogenation mechanism of H2O. Water decomposition was found favorable on O-covered Pd (1 0 0) surface (0.49eV), in agreement with the experimental observations. This result indicated that the joining of Oads could reduce the barrier energy and facilitate the decomposition of H2O. Besides, the distinct differences over Pd (1 1 1) and Pd (1 0 0) surface implied that water decomposition over Pd-based catalysts is a structure-sensitive reaction.

Kinetic evaluation of direct NO decomposition and NO–CO reaction over dendrimer-derived bimetallic Ir–Au/Al2O3 catalysts

The effects of Au on the catalytic performance of Ir metal towards NO reduction by CO and direct NO decomposition have been investigated over dendrimer-derived Ir–Au bimetallic catalyst. Results were also compared to a catalyst prepared by conventional incipient wetness impregnation of metal salts. Characterization of Ir–Au bimetallic samples using scanning transmission electron microscopy, H2 chemisorption and temperature programmed desorption of NO reveals that highly dispersed Ir–Au particles can be obtained by dendrimer templating method. In-situ transmission Fourier transform infrared spectroscopy following adsorption of probe molecules (i.e., CO and NO) on the dendrimer-derived Ir–Au bimetallic sample indicated the presence of electron transfer from Ir to Au sites. In contrast, a geometric effect of Au on Ir was observed over the conventional-derived sample, consistent with X-ray photoelectron spectroscopy measurements. These properties influence the catalytic performance, with higher selectivity towards N2O and a better intrinsic catalytic turnover frequency for reduction of NO by CO and NO decomposition observed for the dendrimer-derived sample. Overall, the results suggest that both NO–CO and NO decomposition reaction are structure-sensitive reactions.

Stellated Ag-Pt bimetallic nanoparticles: An effective platform for catalytic activity tuning

The usefulness of Pt-based nanomaterials for catalysis can be greatly enhanced by coupling morphology engineering to the strategic presence of a second or even third metal. Here we demonstrate the design and preparation of stellated Ag-Pt bimetallic nanoparticles where significant activity difference between the methanol oxidation reaction (MOR) and the oxygen reduction reaction (ORR) may be realized by relegating Ag to the core or by hollowing out the core. In particular the stellated Pt surface, with an abundance of steps, edges, corner atoms, and {111} facets, is highly effective for the ORR but is ineffective for MOR. MOR activity is only observed in the presence of a Ag core through electronic coupling to the stellated Pt shell. The bimetallic Ag-Pt stellates therefore demonstrate the feasibility of tuning a Pt surface for two very different structure sensitive catalytic reactions. Stellated bimetallics may therefore be an effective platform for highly tunable catalyst designs.

Synthesis of Pt Nanoparticles in Water-in-Oil Microemulsion: Effect of HCl on Their Surface Structure

The synthesis of shape-controlled nanoparticles is currently a hot research topic. However, from an applied point of view, there is still a lack of easy, cheap, and scalable methodologies. In this communication we report, for the first time, the synthesis of cubic platinum nanoparticles with a very high yield using a water-in-oil microemulsion method, which unlike others, such as the colloidal method, fulfills the previous requirements. This shape/surface structure control is determined by the concentration of HCl in the water phase of the microemulsion. The results reported here show that the optimal HCl percentage in the water phase is about 25% to obtain the highest amount of cubic nanostructures. Ammonia electro-oxidation is used as a surface structure sensitive reaction to illustrate HCl surface structure effects. Moreover, in situ electrochemical characterization has been performed to study the nanoparticle surface structure.

Shape-Controlled Synthesis of Palladium Single-Crystalline Nanoparticles: The Effect of HCl Oxidative Etching and Facet-Dependent Catalytic Properties

This paper reports a convenient and facile preparation of single-crystalline palladium with controllable shape based on the reduction of kinetic control via hydrochloric acid oxidative etching. The concentration of HCl added to the reaction solution was found to be crucial for the shape evolution of palladium nanocrystals from nanocubes bounded by {100} facets to octahedrons by {111} facets. Palladium nanocubes can be readily obtained at a fast reduction rate without the involvement of additional HCl. With the introduction of a certain amount of HCl to the precursor solution, truncated nanocubes with {111} facets were formed, and the increase of HCl led to the slower reduction rate and the formation of palladium cuboctahedrons enclosed by six {100} facets and eight {111} facets at the expense of gradual shrinkage of {100} facets. The probable mechanism of morphological transformation was proposed upon a batch of experiments. The shape-dependent catalytic performances of as-obtained palladium nanocrystals were investigated by the structure-sensitive reaction of formic acid oxidation. It was found that catalytic activities of palladium nanocrystals displayed a strong dependence on the facets exposed on the surface, and cubic palladium exhibited the best catalytic performance compared with cubooctahedral and octahedral palladium nanocrystals.

Magnesia-supported gold nanoparticles as efficient catalysts for oxidative esterification of aldehydes or alcohols with methanol to methyl esters

Magnesia-supported gold nanoparticles were found to be highly efficient catalysts for the oxidative esterification of methacrolein (MAL) with methanol in the presence of molecular oxygen into methyl methacrylate (MMA) under liquid base-free conditions. MAL conversion of 98% and MMA selectivity of 99% were obtained over the Au/MgO catalyst at 343K after a 2h reaction. Besides the Au nanoparticles, the support also played pivotal roles in the oxidative esterification of MAL. The support with higher density of basic sites, particularly stronger basic sites, showed better performances for the formation of MMA. The enhancement of the intermediate formation by the basic sites is proposed to be the key reason for the superior activity of the Au/MgO catalyst. Our studies on the size effect of Au nanoparticles reveal that smaller Au nanoparticles favor the transformation of MAL, and the turnover frequency increases with decreasing mean size of Au nanoparticles. This suggests that the Au-catalyzed oxidative esterification of MAL is a structure-sensitive reaction. We have demonstrated that the Au/MgO catalyst is also applicable to the oxidative esterification of different aldehydes and alcohols.

Density functional theory study on direct catalytic decomposition of ammonia on Pd (1 1 1) surface

The adsorption and step dehydrogenation mechanism of NH3 on Pd (111) have been studied using density functional theory (DFT) together with periodic slab models. According to the optimized structural and energetic properties, it was found that NH3 and N2 prefer to adsorb on the top site, whereas NH, N, H prefer to adsorb on the fcc site and NH2 prefers on the bri site. In addition, this work identified the optimum configurations for the stable co-adsorption configurations of NHx+H, and N+N. Finally, three transition states were found for analyzing the mechanism of dehydrogenation of NH3, and the N recombination reaction was also considered. The results show that NH is the most abundant intermediate on Pd (111) surface and the dehydrogenation of NH3 is the rate-determining step in the overall reaction. The distinct differences over Pd (111) and Pd (100) surface imply that ammonia decomposition over Pd-based catalyst is a structure-sensitive reaction.

The Partial Hydrogenation of 1,3‐Dienes Catalysed by Soluble Transition‐Metal Nanoparticles

AbstractThe partial hydrogenation of 1,3‐dienes is a structure sensitive reaction that is typically catalysed by classical heterogeneous (heterotopic) or homogeneous (homotopic) catalysts. Recently, soluble transition‐metal nanoparticles (M‐NPs), particularly palladium and gold‐based systems, have emerged as an efficient alternative. Here, we review the current state of the techniques for the partial hydrogenation of 1,3‐dienes by M‐NPs and conclude that, from the reactivity point of view, these materials possess heterotopic‐like and homotopic‐like characteristics. They are heterotopic‐like because the relative concentration of the monoalkene with respect to the diene does not affect the product selectivity and their catalytic performance is affected by their physical properties (such as size and shape). Furthermore, they are easily recoverable, with long catalytic lifetimes. Additionally, as homotopic systems, their reactivity can be tuned by using an appropriate organic stabiliser, which displays substrate‐selective levels that are not observed for classical heterotopic catalysts.

Nanostructure effects on the kinetics and deactivation at reactions over multifunctional catalysts

This study brings into focus some nanostructure-size effects on the deactivation kinetics at reactions over supported multifunctional catalysts. The active phase dispersion as nano-sized islands on the support predetermines diversities in the action of active sites depending on their location and structure. In view of this, the applied approach assumes participation of active site types differing by coordination, configuration and contribution to various reaction routes. The suggested model concerns regularities associated with the availability of active surface atoms in proper arrangements which facilitate structure-sensitive reactions. Problems linking the vulnerability of active sites with their geometry and structure are put to discussion. Furthermore, the model relates the probabilities for action of different site types to the size of active-phase islands. The effect of site blockage on the probabilities for existence of multi-site active centers (catalytic clusters) facilitating structure-sensitive reactions is explored. The performed analysis points out that, when matter concerns reactions facilitated by multi-centered active sites, two distinct reasons can be specified, by virtue of which coke species may affect the activity of catalytic clusters: (i) canceling the action of partially or totally coke-covered cluster configurations and (ii) reducing the capability of the adjacent active atoms to construct multi-centered active configurations.

Design Principles of Heteroepitaxial Bimetallic Catalysts

Heteroepitaxial bimetallic catalytic materials have been attracting considerable interest because of their unique properties and applications in energy, chemicals, and fuel sectors. Optimizing the activity of bimetallic catalysts requires the ability to understand and precisely manipulate the nanostructure (loading and dispersion of an admetal on a host metal). Ammonia decomposition catalyzed by Ni on Pt(111) [Ni/Pt(111)] is a prototype structure sensitive reaction for this purpose, in which both N–H bond breaking and N–N bond formation can be rate-limiting steps. Here, a first-principles kinetic Monte Carlo simulation framework reveals the bifunctional role of Ni clusters on Pt(111) in NH3 decomposition: dehydrogenation on Ni terrace and N2 desorption near Ni steps on Pt. We show that the dynamic interplay between surface nitrogen diffusion and association near the steps of Ni clusters and Pt terraces is responsible for the lower than expected nitrogen desorption temperature found in experiments. Our res...

Theoretical Study of H2O Adsorption on Zn2GeO4 Surfaces: Effects of Surface State and Structure–Activity Relationships

We employed the density functional theory to investigate the interaction of H2O with Zn2GeO4 surfaces, considering both perfect and defective surfaces. The results revealed that the interaction of H2O with Zn2GeO4 surfaces was dependent on the structure of the latter. For perfect surfaces, H2O adsorbed at the Ge3c···O2c site of a (010) surface could spontaneously dissociate into an H atom and an OH group, whereas H2O tended to adsorb at the O2c-M3c-O3c site of a (001) surface by molecular adsorption. The presence of oxygen defects was found to strongly promote H2O dissociation on the (010) surface. Analysis of the surface electronic structure showed a large density of Ge states at the top of the valence band for both perfect and defective (010) surfaces, which is an important factor affecting H2O dissociation. In contrast, perfect and defective (001) surfaces with surface Ge states buried inside the valence band were significantly less reactive, and H2O was adsorbed on these surfaces in the molecular form. This information about the adsorbate geometries, catalytic activity of various surface sites, specific electronic structure of surface Ge atoms, and their relevance to surface structure will be useful for the future design of the Zn2GeO4 photocatalyst, as well as for the atomistic-level understanding of other structure-sensitive reactions.

Pd on boron-doped hollow carbon spheres – PdO particle size and support effects

Boron-doped hollow carbon spheres (B-HCSs) were synthesized using a CVD injection method (BCl3 in toluene; Ar; 900°C; 4h) using Stöber silica spheres as template. A Pd complex was loaded onto the B-HCSs using deposition and impregnation methods, and the materials were studied by XRD, SEM, TEM, Raman spectroscopy, TGA, ICP-OES, XPS, and 11B solid-state NMR spectroscopy. The boron-doped carbon-supported Pd catalysts (Pd/B-HCS) were compared with a Pd-loaded boron-doped carbon nanotube (Pd/B-CNT) catalyst (both have a hollow interior) in the solvent-free oxidation of alcohols using oxygen as an oxidant at 125°C under base-free conditions. The Pd particle size was varied (2.5–12nm) by changing the Pd/B-HCS calcination temperature (200–550°C), and this affected the activity but not the selectivity of the benzyl alcohol to benzaldehyde reaction. The data revealed the key role of the Pd particle size on the reaction that was influenced by the B–Pd–C interaction. The reaction rate depended on the mean size of Pd particles and showed a maximum when catalysts were calcined at 300°C, revealing that the aerobic oxidation of benzyl alcohol catalyzed by the supported PdO (dPd>2.5nm) nanoparticles was not a structure sensitive reaction.

Catalytic decomposition of hydrazine on ruthenium catalysts in nitric acid solutions

The kinetics and stoichiometry of catalytic decomposition of hydrazine on 5% Ru/SiO2 in nitric acid solutions were studied. The major reaction products are N2, N2O, and HN3. Ammonium ion is not formed. HN3 is generated owing to the occurrence of a heterogeneous-catalytic redox cycle on the catalyst surface. The influence exerted on the reaction kinetics by reaction mixture composition and temperature was examined. The process scheme was suggested. The hydrazine decomposition on ruthenium catalysts is a structure-sensitive reaction (positive size effect).

Water Adsorption and Decomposition on N/V-Doped Anatase TiO2 (101) Surfaces

With use of the generalized gradient approximation within density functional theory approach combined with periodic slab models, the adsorption and decomposition behaviors of water molecule on N/V-monodoped, and N–V-codoped anatase TiO2 (101) surfaces at gas ambient were studied. By optimizing the geometrical configurations of initial molecular adsorption states, transition states, and final dissociative adsorption states, the adsorption energy of each species and the reaction activation energy data on the reaction pathways were obtained. On N-doped surface, the decomposition reaction pathway of water is changed: the activation energy is reduced and the decomposition reaction is an exothermic reaction, which is favorable for the decomposition of water. On the other hand, V-doped surface is unfavorable for water decomposition. The decomposition reaction of water on the N–V-codoped surface is a structure-sensitive reaction process, i.e., it has a completely different reaction activity on the different surfa...

ChemInform Abstract: Monodisperse Metal Nanoparticle Catalysts: Synthesis, Characterizations, and Molecular Studies under Reaction Conditions

AbstractReview: 106 refs.

Synthesis and performance of highly dispersed Cu/SiO2 catalysts for the hydrogenolysis of glycerol

The performance of Cu/SiO2 (commercial silica gel, SBA-15 and SBA-15 treated at 900°C) catalysts for the hydrogenolysis of glycerol to propylene glycol is investigated with emphasis on the stability characteristics. Cu catalysts with large crystals, small monodisperse crystallites or a highly dispersed XRD amorphous copper phase were obtained after calcination in stagnant air, in a flow of NO/N2 or a flow of air, respectively. Analysis by XRD, N2O surface oxidation and TEM confirmed the variation of the Cu specific surface area by the calcination conditions and the type of silica support used. The different dispersion characteristics resulted in different activities (20–50% glycerol conversion), while all the catalysts proved to be highly selective towards propylene glycol (92–97%). Present results indicate that glycerol hydrogenolysis over Cu-based catalysts is a structure sensitive reaction as significant variations in initial TOF were observed as a function of varying Cu crystallites. It is shown here that the presence of a solvent greatly influences the intrinsic reaction rate and the nature of structure sensitivity. The deactivation behaviour of all catalysts was studied, and based on detailed characterization of the spent samples it was attributed to Cu sintering and the presence of strongly adsorbed species on the catalytic surface. The 18wt%Cu/silica gel (air) catalyst presented only moderate deactivation (∼20%) while the catalyst supported on SBA-15 calcined at 900°C (SBA900C) proved to be the most stable with negligible deactivation after three consecutive runs.

Metal Nanoparticle Catalysts Beginning to Shape-up

The field of heterogeneous catalysis has received a remarkable amount of interest from scientific and industrial perspectives because of its enormous impact on the world's economy: more than 90% of chemical manufacturing processes use catalysts. Catalysts are also essential in converting hazardous waste into less harmful products (car exhaust) and in generating power (fuel cells). Yet in all applications, it remains a challenge to design long lasting, highly active, selective, and environmentally friendly catalytic materials and processes, ideally based on Earth-abundant elements. In addition, the field needs more satisfactory experimental and theoretical approaches to minimize trial and error experiments in catalyst development. Nanocatalysis is one area that is developing rapidly. Researchers have reported striking novel catalytic properties, including greatly enhanced reactivities and selectivities, for nanocatalysts compared to their bulk counterparts. Fully harnessing the power of nanocatalysts requires detailed understanding of the origin of their enhanced performance at the atomic level, which in turn requires fundamental knowledge of the geometric and electronic structures of these complex systems. Numerous studies report on the properties that affect the catalytic performance of metal naoparticles (NPs) such as their size, interaction with their support, and their oxidation state. Much less research elucidates the role played by the NP shape. Complicating the analysis is that the preceding parameters are not independent, since NP size and support will affect which NP shapes are most stable. In addition, we must consider the dynamic nature of NP catalysts and their response to the environment, since the working state of a NP catalyst might not be the state in which the catalyst was prepared, but rather a structural and/or chemical isomer that responded to the particular reaction conditions. In order to address the complexity of real-world catalysts, researchers must undertake a synergistic approach, taking advantage of a variety of in situ and operando experimental methods. With the continuous shrinking of the scale of material systems, researchers require more sensitive experimental probes and computational approaches that work across a wide range of temperatures and chemical environments. This Account provides examples of recent advances in the preparation and characterization of NP catalysts with well-defined shapes. It discusses how to resolve the shape of nanometer-sized catalysts via a combination of microscopy and spectroscopic approaches, and how to follow their evolution in the course of a chemical reaction. Finally, it highlights that, for structure-sensitive reactions, controlled synthesis can tune catalytic properties such as the reaction rates, onset reaction temperature, activity, and selectivity.

Shape-Controlled Synthesis of Pt Nanocrystals: The Role of Metal Carbonyls

Well-controlled synthesis of nanocrystals is necessary to unambiguously correlate the structural properties of nanocrystals with the catalytic properties. The most common low-index surfaces are (111) and (100). Therefore, model materials with {111} and {100} facets are highly desirable, in order to understand the catalytic properties of (111) and (100) surfaces for various structure-sensitive reactions. We report a solution-phase synthesis using metal carbonyls as additives. This synthetic method produces highly monodisperse Pt octahedra and icosahedra as the model of Pt{111}, Pt cubes as the model of Pt{100}, respectively. Several other morphologies, such as truncated cubes, cuboctahedra, spheres, tetrapods, star-shaped octapods, multipods, and hyper-branched structure, are produced, as well. A bifunctional role of metal carbonyl in the synthesis is identified: zerovalent transition metal decomposed from metal carbonyl acts as a shape-directing agent, while CO provides the reducing power. These high-quality shape-controlled Pt nanocrystals are suitable for model catalyst studies.